fill a boil on human skin and you get the basic message: The phages will attack the bugs and multiply as long as there are bacteria left: The drug produces itself in the body until its food – the infection – is gone.

It's as simple and intriguing as that.

Already one of the two discoverers of phages has had this idea. Félix d'Herelle, a bacteriologist working at the Pasteur Institute in Paris, isolated phages in 1916 from the faeces of French soldiers struck by bacillary dysentery.

He observed that the enigmatic microbe always appeared in the stools of patients just before they began to recover from the disease. Further studies showed that these bacteriophages, as he had named them soon, dissolved even the densest cultures of dysentery bacteria (Shigella) in a spectacular manner.

D'Herelle concluded that these phages had something to do with the healing process of bacillary dysentery and he wanted to harness their power for fighting infections. First, he treated chickens with phages against an infectious disease called fowl typhoid. As he had soon discovered the picky appetite of phages he isolated for these experiments specific phages against Salmonella gallinarum, the agent of fowl typhoid.

In 1919, he thought he had done enough research and decided to start treating humans. D'Herelle's first patients were five children with bacillary dysentery. They were all cured. If the phages really were responsible remains unclear as such anecdotal evidence can never constitute a proof.

Nontheless, these pioneering experiments were the beginning of phage therapy. In the next decades, it was used in many countries to treat millions of patients against infectious disesases like cholera, bacillary dysentery and furuncles. But once doctors ushered in penicillin in the 1940s, phage therapy fell by the wayside in most places.

Only in the Eastern bloc it survived alongside antibiotics. After the fall of the Soviet Union, western scientists took note of the parallel medicine from behind the wall – their interest boosted by the growing problem of resistance.

Phage therapy has several advantages over antibiotics. One of them is that phages are very specific and do not harm the useful bacteria that live in and on the body. Antibiotics, however, also attack harmless bacteria that make a living on us. This can, for instance, lead to severe diarrhoea, especially in patients with a weak immune system. Furthermore, phages attack also bacteria that are resistant against antibiotics.

However, there are also drawbacks of phage therapy. Probably the strongest is that, despite its long history, the proponents of phage therapy never succeded in producing formal proof of efficacy in human treatment. There are many studies with positive results. But they all have one or the other weak point making definitve conclusions difficult. There are several very good studies in animals that do show conclusively that phages can eradicate infections in the body.